1. Field of the Invention
This invention relates to zoom lenses in which the first lens group, counting from the front, is adjustable for focusing and more particularly to a zoom lens of high standard optical performance in which the first lens is group reduced in outer diameter to improve the compactness of the lens system.
2. Description of the Prior Art
In general, it is desirable that zoom lenses be capable of closeup photography and of taking the compact form. This is because a close object can be advantageously imaged over the entire area of the picture frame, and because the use of the compact form lightens the weight of the camera and improves its manageability. In the prinicipal types of zoom lenses, an extension of the focusing range to closer object distances calls for an increase in the total forward movement of the focusing lens group. The result is that the outer diameter of the focusing group must be correspondingly increased, since the light ray from the maximum possible half-angle of view field passes through the lens surfaces of the first lens group at increasing heights from the optical axis, as the total forward movement of the focusing lens group increases.
The maximum diameter of all of the lens groups constituting a zoom lens occurs, in many cases, at the first group arranged for the focusing function. To facilitate an improvement of the compactness of the lens, therefore, it is required to reduce the outer diameter of the first lens group. In general is it true that when the power of every lens group constituting the zoom lens is strengthened, the outer diameter of the first lens group can be reduced to some extent. As the power of each lens group increases, however, the variation of the aberrations upon zooming is increased, giving rise to a serious problem when in practical use. Therefore, a much desired increase in the power of each lens group cannot be made. An alternate solution is that a particular type of zoom lens suited for a minimization of the outer diameter of the first lens group is employed, comprising two groups of opposite power, the negative one of which precedes the positive. Since this type of lens is advantageous in achieving a good state of correction of aberrations on the wide angle side, it has found wide use in wide-angle zoom lenses. But, as the zoom ratio increases, the high grade imagery is made more difficult to preserve at the telephoto settings, and further on the point of view of the aberrational problem, it is difficult to be compatible with an increase of the relative aperture.
As an effective method of minimizing the outer diameter of the first lens group, mention may be made of that disclosed in U.S. Pat. No. 4,159,865 where the stop is made movable along with the zoom groups, with an advantage that the bulk and size of the zoom lens as a whole is reduced.